Dreaming. My cats do that all the time. Sometimes their whiskers or front paws twitch too. Don't verbs about it unless you see signs of stress like change contained by diet, activity level, lethargy, etc.
If you are truly concerned you can try calmly touching the hind legs, hips or back to see if there is sensetivity or niggle. Also examine for bug bites, animal bites, or scratches. Anything funny like that I would go to the Vet.

Answers: don't be embarrassed hes just dreaming of chasing mice! ur not alone one time my cat woke him self up from meowing in his sleep and when he did he took rotten running like something terrible was after him it be LOL funny

Sometimes animals twitch in their sleep.
"Twitches of the limbs during REM sleep contained by adult mammals result from descending motor activation from the brainstem." http://cat.inist.fr/?aModele=afficheN&cp... Maybe that's what happens with animals, too.
no he's just dreaming or chasing mice

No nought bad my 3 cats do that when they sleep sometimes, they might be dreaming there running or playing. Kinda like how culture talk in there sleep or sleep pace. Its nothing to worry about.
This is inborn for a cat to do while restind or sleeping, I have seen many useless visit to me at the vets office for that. He is having a dream in the region of chasing something or other things. It would only be bad if he be noit eating or going to the bathroom.!